Head and Neck Squamous Cell Carcinoma (SCCHN) is one of the most common cancers worldwide and a significant cause of cancer morbidity and deaths. Although SCCHN is unique among solid tumors in that the majority of patients present with local disease, metastasis remains a critical problem for SCCHN treatment: patients without metastases can be cured; patients with metastases cannot. Even for local disease, despite improvements in outcome with intensive, multimodality therapies, such treatments are highly toxic, poorly tolerated and only partially efficacious. Long-term survival is only ~ 50%, with most patients dying from their cancers. There is an urgent need to uncover the critical genetic alterations in SCCHN and to determine how these impact patient outcome and metastatic risk. To search for genes that are critical for head and neck squamous carcinoma (SCCHN) biology, prognosis and therapy, I have initially used high resolution single nucleotide polymorphism (SNP) arrays to detect copy number alterations in 684 human cancer cell line of diverse tissue origin, including SCCHN. This analysis has identified recurrent homozygous deletions involving >200 genes. Among these, PARD3, encoding a critical regulator of cell polarity, is disrupted in squamous carcinoma cell lines and primary tumors, including from the head and neck region. Since abnormal cell polarity has been linked to increased migration and metastasis, these findings may implicate loss of function of PARD3 or its downstream effectors in the risks of recurrence and metastasis of a subset of these cancers. The overall goals of this proposal are: (1) to further characterize the role of PARD3 in SCCHN tumorigenesis, migration and metastasis; and (2) to capture and genotype circulating tumor cells (CTCs) from the peripheral blood of patients with SCCHN, to determine if CTCs can identify patients at risk for metastases before metastatic disease is clinically apparent. The experiments will utilize primary tumors and peripheral blood from patients treated for SCCHN, as well as PARD3 loss-of-function and gain-of-function squamous cell lines for in vitro and in vivo studies to identify the cancer-related phenotype(s) controlled by PARD3 and the domains and activities required for tumor suppression. By characterizing PARD3 loss in the tumor type in which inactivating mutations have arisen as a primary event during tumor formation, this work will directly implicate alterations in cell polarity in the development and invasive and metastatic potential of these cancers. By capturing and molecularly characterizing CTCs in actual SCCHN patients undergoing treatment, this work will permit the real time in vivo analysis of tumor cells to quantify clinical risk and guide treatment. As a result, these studies may lead to improved biomarkers and suggest potential new targets for therapies, thus providing benefit to public health by improving the care of patients with these cancers.